1. Field of the Invention
The present invention relates to a magnetooptic device to control a degree of a magnetooptic effect by applying an electric field and to a method of driving such a device. The magnetooptic device of the invention is suitably used as, for instance, an optical modulator or an optical isolator in the field of optical communication or an optical memory.
2. Related Background Art
In recent years, optical fiber communication using a semiconductor laser or the like as a light source and an optical memory using an optical disk, a magnetooptic disk, or the like have been developed in association with the realization of a high transmission speed of information and a large recording capacity of information. Those systems need an optical device having a variety of functions.
For instance, in the optical communication such as an optical fiber communication or the like, a system using an external optical modulator to further increase a bit rate is studied. According to such a system, since the semiconductor laser is DC driven, an optical pulse train of a narrow spectrum width and a high quality can be obtained.
As specifications of such an optical modulator, it is required that a high speed modulation of about 10 Gbps can be performed and a large modulation amplitude of 10 dB or more is provided. An optical modulator having a structure which satisfies such specifications and is suitable for further miniaturization and high integration is now being developed. However, the development is still insufficient.
On the other hand, in the above optical communication and optical memory, an instability of the oscillation and noises due to the return light of the semiconductor laser always cause a problem. An optical isolator is known as only one device to completely prevent that such a return light enters the semiconductor laser. High integration and miniaturization of the optical isolator and the semiconductor laser are now being studied. However, there are still several problems with respect to such a study.
A conventional example of the above optical modulator and the optical isolator will now be further practically described hereinbelow.
An optical modulator using a quantum confined Stark effect (QCSE) has been proposed in "Quantum Electron", IEEE J., Vol. QE-21, page 117, 1985, "Quantum Electron", IEEE J., Vol. QE-21, page 1462, 1985, or the like. FIG. 1 shows a schematic cross-sectional view of an example of such an optical modulator.
In FIG. 1, an n.sup.+ -AlGaAs layer 62, an intrinsic (i-) multiple quantum well (MQW) layer 63, and a p.sup.+ -AlGaAs layer 64 are sequentially formed as films on an n.sup.+ -GaAs substrate 61. A hole to transmit the light is formed in the substrate 61 by a selective etching process. Electrodes 66 and 65 are formed under the lower surface of the substrate 61 and on the p.sup.+ -AlGaAs layer 64, respectively.
The i-MQW layer 63 is constructed by alternately laminating a GaAs layer as a well layer and an AlGaAs layer as a barrier layer. When an inverse-bias voltage is applied to a pin structure constructed as mentioned above, as shown in FIG. 2, as a value of the applied voltage increases, an exciton absorption peak decreases and an absorption edge moves to the long wavelength side, that is, what is called a red-shift occurs.
Therefore, as shown in A or B in FIG. 2, by setting a wavelength of an input light to the device to a predetermined value and by changing the voltage which is applied to the device, a transmittance of the input light of a set wavelength .lambda. can be changed as shown in FIGS. 3A to 3D. The optical modulator of FIG. 1 intends to modulate a transmission light amount by using such an effect.
On the other hand, a device which is constructed by integrating a semiconductor laser and an external optical modulator has been proposed in "Japan J. Applied Physics", Vol. 24, L442, 1985. FIG. 4 shows a schematic perspective view of such device.
In FIG. 4, reference numeral 77 denotes an n.sup.+ -GaAs substrate; 76 an n.sup.+ -GaAs layer; 75 an n-Al.sub.y Ga.sub.1-y As layer (y=0.3); 79 an MQW layer; 74 a p-Al.sub.y Ga.sub.1-y As layer (y=0.3); 73 an Si.sub.3 N.sub.4 layer; 72 a p.sup.+ -GaAs layer; 78 an n-type electrode; and 71 a p-type electrode. A stripe-shaped optical waveguide is formed by mesa-etching the upper portion of the n.sup.+ -Al.sub.y Ga.sub.1-y As layer 75. The device is separated into a laser section 91 and a modulator section 92 by a groove 93.
As mentioned above, the laser section 91 and the modulator section 92 have the same pin structure. The waveguides of those sections are optically coupled. Therefore, the light emitted from the laser section 91 enters the waveguide of the modulator section 92 and is waveguided by a predetermined distance and, thereafter, it is emitted. At this time, by applying an inverse-bias voltage to the modulator section 92, the waveguided light is absorbed by the foregoing QCSE in accordance with the applied voltage value. That is, an intensity of the light which is emitted from the modulator section 92 can be modulated by the control of the applied voltage.
FIG. 5 shows a conventional example of the optical isolator. A material having a large magnetooptic effect such as a paramagnetic glass 101 is put into a hollow cylindrical permanent magnet 102 and a magnetostatic field H is applied to the glass 101. A laser beam 105 which becomes a linear polarization light by passing through a polarizing plate 104 is input to the glass 101. In this case, a polarizing direction of the laser beam 105 is rotated by 45.degree. by a Faraday effect when the laser beam transmits the paramagnetic glass 101.
A polarizing plate 103 on the emission light side is arranged so that an angle between a main axis of the polarizing plate 103 and a main axis of the polarizing plate 104 on the incident light side is set to 45.degree.. Therefore, the laser beam 105 whose polarizing direction was rotated as mentioned above directly passes through the polarizing plate 103. On the other hand, the polarizing direction of the return light which enters from the side of the polarizing plate 103 is also rotated for a period of time when the return light passes through the polarizing plate 103 and reversely moves in the paramagnetic glass 101. An angle between the polarizing direction of the return light and the main axis of the polarizing plate 104 on the incident light side is set to 90.degree.. Thus, the return light cannot pass through the polarizing plate 104 on the incident side and is cut.
An isolation ratio (of a degree such as to cut the return light) of the device depends on the Faraday rotation angle. The Faraday rotation angle .theta..sub.F is given by EQU .theta..sub.F =VlH
when the light passes through a material such as an above paramagnetic glass having a length of l. In the above equation, V denotes a Verdet's constant and H indicates a magnitude of an external magnetic field. At this time, to set the angle .theta..sub.F to 45.degree. in order to maximize the isolation ratio, it is necessary to accurately adjust the length l of the paramagnetic glass 101 and the magnetostatic field H by the permanent magnet 102.
However, the above conventional device has the following problems.
First, there is a problem such that the modulation amplitude of the optical modulator as shown in FIGS. 1 to 4 is shallow. For example, the modulation amplitude is merely about 50% (3 dB) as shown in FIGS. 3A to 3D. In the case of using the optical modulator for the optical communication or the like, a modulation amplitude of at least 10 dB or more is generally necessary. Therefore, it is a present situation that the foregoing optical modulator is not used as a practical device.
There is also a problem such that the optical isolator as shown in FIG. 5 must be manufactured at a high dimensional accuracy by using a material having a large magnetooptic effect. It is also necessary to accurately control the external magnetic field. That is, the values of l and H must be accurately set so as to set the Faraday rotation angle .theta..sub.F to 45.degree. at a high precision.